Corrosion of iron by air yields rust, or iron oxides such as goethite (.varies.-FeO (OH)), hematite (Fe.sub.2 O.sub.3) and magnetite (Fe.sub.3 O.sub.4). Exposure of these iron oxides to hydrogen sulfide rich conditions where oxygen content is low results in a sulfidation reaction which yields mackinawite (FeS.sub.x) which can form greigite (Fe.sub.3 S.sub.4) and/or pyrite (FeS.sub.2). This sulfidation step is exothermic. Oxidation of these iron sulfides, as by exposures to air or oxygen rich conditions, is highly exothermic and can result in pyrophoric activity.
Hydrogen sulfide is often present in crude oil and can react with iron oxides formed in transportation, processing or storage vessels. Exposure of the resulting iron sulfides to air can result in pyrophoric activity and a potentially explosive situation. For example, the reaction of hydrogen sulfide with iron oxides present in oil tankers in the area above the liquid crude oil can result in the formation of pyrophoric iron sulfides. Upon discharge of the crude oil, exposure of the iron sulfides to air can result in pyrophoric activity in the head space and explosive results are possible. Similar conditions can exist in other crude oil handling, transporting or processing vessels. In particular, pyrophoric iron sulfides have been found in refinery units, sour water strippers and amine units in addition to oil tankers. These units have reducing atmospheres. When these units are opened up, as for repair or maintenance, exposure to air gives rise to the possibility for the pyrophoric iron sulfides to ignite flammable vapors that are still in the units.
The reactions involved in the formation of iron sulfide and its subsequent oxidation on exposure to oxygen may be represented in a simplified form as follows:
Sulfidation Reaction
Fe.sub.2 O.sub.3 +3 H.sub.2 S.fwdarw.2 FeS+S+3H.sub.2 O PA1 4 FeS+3 O.sub.2.fwdarw.2 Fe.sub.2 O.sub.3 +4 S
Oxidation Reaction
Both of these reactions are exothermic with enthalpy, .DELTA.H, values of -168 and -635 kJ/mol, respectively. If the oxidation reaction is allowed to proceed rapidly with little dissipation of heat, high temperatures leading to glowing and sparking can be expected in the material.
Russian Patent No. 1,449,138 discloses the use of polymer/ionomers containing amide and carboxylate groups to prevent the spontaneous combustion of pyrophoric deposits of iron sulfide. The disclosed method comprises contacting pyrophoric deposits of iron sulfide with an aqueous solution of a deactivating solution of a polymer-ionomer containing amide and carboxylate groups.